Acoustic resonators are used to filter electrical signals in various electronic applications. For example, acoustic resonators are used as bandpass filters in cellular phones, global positioning system (GPS) devices, and imaging applications, to name but a few.
An acoustic resonator can be characterized generally by a center frequency and bandwidth. However, due to a variety of intrinsic and extrinsic influences, the center frequency and bandwidth can drift over time—a process referred to as frequency drift, or more generally “aging.”
One cause of aging in acoustic resonators is physical stress. Physical stress can be caused, for example, by forces transmitted to the acoustic resonator through adjacent components. As an example, an acoustic resonator can be mounted on a printed circuit board (PCB) comprising metal and laminate components. As the PCB is heated or cooled, the PCB may expand or contract unevenly because the metal and laminate components have different temperature coefficients of expansion. This uneven expansion or contraction can cause the PCB to change shape in a “potato chip” fashion. As the PCB changes shape, the PCB can transfer forces to the acoustic resonator through various intervening components, such as an epoxy bonding material or a silicon microcap. As these forces are transferred to the acoustic resonator, they will change the center frequency of the acoustic resonator. Although the frequency change is relatively small, it is significant in terms of other sources of aging such as the electrode metal relaxation effect associated with quartz crystal aging.
FIG. 1A is a diagram illustrating forces applied to a conventional acoustic resonator structure 100. For explanation purposes, it will be assumed that acoustic resonator structure 100 is located in a chip scale package mounted on a PCB. Forces are applied to the package from the PCB, and from the package to acoustic resonator structure 100 as indicated by arrows in FIG. 1A.
The forces shown in FIG. 1A can originate from various sources. For example, forces can originate from a PCB that has been warped in response to temperature changes, as described above. Alternatively, forces could originate from a PCB that has been bent when clamped to a chassis or another motherboard, or from the soldering of the package onto a PCB.
Referring to FIG. 1A, acoustic resonator structure 100 comprises a silicon substrate 105 located within the package, and a film bulk acoustic resonator (FBAR) 115 formed on substrate 105. An air gap 110 is formed between substrate 105 and FBAR 115 so that FBAR 115 can resonate freely.
Curved lines 120 represent the interface of the mounted resonator shown with other structures such as a printed circuit (pc) board, packaging, etc. Forces created by, or presented to these structures can be present. These forces can be transferred from the package to substrate 105 through various intervening features, such as an epoxy bonding or microcap structure (not shown). The transferred forces create stresses 125 on substrate 105. Stresses 125 propagate through substrate 105 and other features to create stresses 130 where FBAR 115 is connected to substrate 105. Stresses on 130 exert torque on FBAR 115, which can change the center frequency on the FBAR 115.
FIG. 1B is a diagram illustrating a simulation of forces transferred from substrate 105 to FBAR 115. As illustrated in FIG. 2, the forces on substrate 105 cause stress at an edge of FBAR 115. The stress is transmitted horizontally through FBAR 115, which can affect the resonance of the FBAR 115, as explained above.
FIG. 2 is a graph illustrating changes of the center frequency of a conventional acoustic resonator structure as a function of temperature. These changes are caused by stresses on the acoustic resonator due to the changing temperature. The graph of FIG. 2 was generated with a so-called zero drift resonator (ZDR) mounted on a PCB in laboratory conditions. A resonator under real-life conditions may experience even more frequency drift than that illustrated in FIG. 2.
Referring to FIG. 2, the ZDR was heated from an initial temperature of approximately 70° C. to a temperature of approximately 130° C. The resonator was then cooled to approximately 25° C. and heated back to approximately 70° C. The center frequency of the acoustic resonator changed by approximately −50 ppm when the temperature was raised from 70° C. to 130° C. Then, as the temperature was cooled back to 70° C., the center frequency passed through a point at 0 ppm, which is offset from the original center frequency by approximately 20 ppm. As illustrated by the different center frequencies exhibited at 70° C., the center frequency of the acoustic resonator exhibits both temperature dependence as well as temperature based hysteresis. The parabolic temperature dependence is a property of the stiffness of the materials present in the acoustic stack of the ZDR and can be compensated elsewhere in the circuit. But, the hysteresis is created by variations in applied forces to the substrate. One cause for the change in force is that the epoxy (a hydrophilic material) outgases moisture and as the epoxy becomes more desiccated, it shrinks and thus applies a different force to the mounted ZDR.
The frequency changes shown in FIG. 2 may be too large for certain high accuracy electronic applications. For example, GPS devices can only tolerate aging-related frequency changes on the order of +/−0.5 ppm. Similarly, wireless applications, such as low power radios used in WiFi or Bluetooth can only tolerate aging-related frequency changes on the order of +/−10 ppm.
What is needed, therefore, are techniques for reducing frequency drift due to physical stresses in acoustic resonator structures.